OLED's are effectively light emitting diodes made from semiconducting organic materials. They are currently still under development and have potential application in numerous fields. First ultra-thin and low-voltage OLEDs have been described in C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes” Appl. Phys. Lett., Vol. 51, pp. 913-915 (1987). Since then, much development has been made to improve these devices for applications in flat panel displays as well as in solid state lighting.
A typical OLED is composed of a layer of organic materials situated between two electrodes, the anode and cathode, all deposited on a substrate. During operation, a voltage is applied across the OLED such that the anode is positive with respect to the cathode. A current of electrons flows through the device from cathode to anode, as electrons are injected into the LUMO of the organic layer at the cathode and withdrawn from the HOMO at the anode. This latter process may also be described as the injection of electron holes into the HOMO. Electrostatic forces bring the electrons and the holes towards each other and they recombine forming an exciton, a bound state of the electron and hole. This happens closer to the emissive layer, because in organic semiconductors holes may be more mobile than electrons. The decay of this excited state results in a relaxation of the energy levels of the electron, accompanied by emission of radiation whose frequency is in the visible region.
Research on how to improve the device emission efficiency continues to be a major focus. In general, improved efficiency can be achieved through the use of highly efficient luminescent materials and in designing novel device structures. Higher current efficiency can be achieved by multiphoton devices consisting of stacked units of OLEDs. The current efficiency can be multiplied because of electron and hole recycling.
If OLED's with individually addressable areas are manufactured, this is done by a process in which the organic OLED material and the cathode material of each segment is deposited with individual masks in such a way, that only the areas that are supposed to emit light are coated with these materials. This leads to the situation that the masks to coat the second emissive layer area is overlapping or touching the area of the already deposited layers. This can lead to micro damages of these layers and thereby to visual damages or to short circuits. This problem is not limited to OLED devices and may as well occur in other patterened light emitting devices with layered structures.